Featured image and figures used with permission via ACS AuthorChoice open access

As human industries continue to introduce hazardous chemicals into the environment, new methods of bioremediation (the degradation of pollutants by microorganisms like bacteria) may be the best way to clean up our spills. While humans try to store and contain hazardous waste, some bacteria can use this as a food source. The hope is that one day, when we think of symbiotic relationships between humans and organisms, bacteria metabolizing our hazardous waste will be as common to us as trees converting our carbon dioxide back into oxygen for us to breathe.

Two concerning chemicals in the environment today are trichloroethene (TCE) (which is used most frequently in the production of cooling chemicals like those used in air conditioners [1]) and 1,4-dioxane which is often used to stabilize chlorinated solvents [2] such as TCE. Both TCE and 1,4-dioxane are carcinogenic (potentially cancer-causing) in humans, and they are frequently leaked into the environment together, accumulating in the groundwater and poisoning our land.

(left to right) 1,4-dioxane [3], trichloroethene [4], cis-1,2-dichloroethene [5], vinyl chloride [6]TCE can be degraded by bacteria called the “Dehalococcoides,” but these bacteria grow best and degrade TCE in anaerobic (oxygen-lacking) environments. Unfortunately, anaerobic breakdown of TCE results in toxic products called “cis-1,2-dichloroethene” (cDCE) and “vinyl chloride” (VC). Aerobic (in the presence of oxygen) breakdown of TCE can avoid the accumulation of these toxic products, but Dehalococcoide bacteria cannot do this alone without the addition of other chemicals or bacteria.

The current researchers chose to use “KB-1” bacteria (a frequently used mixture of Dehalococcoide bacteria) to break down the TCE. They also needed a bacterial strain that could break down both cDCE and 1,4-dioxane using oxygen. Luckily, bacteria called “Pseudonocardia dioxanivorans CB1190” grow well in oxygen environments, can break cDCE down into non-toxic products, and also use 1,4-dioxane as a food source. The scientists decided to combine these bacteria together and add both TCE and 1,4-dioxane.

The scientists knew that locations where pollutants first enter the groundwater is usually anaerobic, and that pollutants then flow “downstream” to more aerobic locations. Because of this, they grew their bacteria first in flasks with water and nutrients where all the oxygen had been removed to create an anaerobic environment. They then added oxygen, simulating the conditions in aerobic groundwater. In Fig. 1 below, this experimental setup is called the “Anaerobic/Aerobic” condition (Fig. 1A green lines, Fig. 1B yellow lines).

The scientists found that KB-1 cells were indeed able to break TCE down without oxygen, resulting in zero TCE after less than 1 day (Fig. 1A, solid green line). Then, oxygen was added to the system. Although cDCE had been produced as TCE was degraded, it was broken down into nontoxic products after oxygen was added, resulting in zero cDCE after 5 days (Fig. 1A, dashed green line). They also found that, in aerobic conditions, 1,4-dioxane was completely degraded in just 4 days (Fig. 1B, yellow line).

Fig.1. Levels of (A) TCE and cDCE, as well as (B) 1,4-dioxane in anaerobic (gray background) and aerobic (white background) conditions over time. Levels of (A) KB-1 cells (vertical gray bars) and (B) CB1190 cells (vertical gray bars) were also measured at 3 time points.

To see how well the KB-1 cells and the CB1190 cells were growing under anaerobic versus aerobic conditions, the amount of each type of cell was also measured three times: time zero, upon oxygen addition, and once all three toxic chemicals were completely gone. This was done by measuring how much a gene—produced only by one type of cell—was expressed at each time point. A gene called tceA was used to count the number of KB-1 cells in the flasks (Fig. 1A, vertical gray bars), and a gene called dxmB was used to count the CB1190 cells (Fig. 1B, vertical gray bars). Results showed that KB-1 cells grew a lot in anaerobic conditions, but stopped growing after oxygen was added. In contrast, CB1190 cells did not grow much without oxygen, but they were still able to survive, and they grew a lot once oxygen had been added.

Thus, it seemed that KB-1 cells could break TCE down into cDCE under the initial anaerobic conditions, and that CB1190 cells could break down both cDCE and 1,4-dioxane once oxygen was added, getting rid of all three carcinogens. However, in their first experiment, oxygen was added after less than one day, whereas in the real environment, the CB1190 cells may have to survive without oxygen for longer. The scientists therefore tested the ability of these cells to still degrade 1,4-dioxane after increasing lengths of time without oxygen. In all conditions tested, the CB1190 cells survived oxygen deprivation—even up to 35 days without it—and were able to degrade 1,4-dioxane completely (Fig. 2).

Fig. 2. Levels of 1,4-dioxane in the presence of CB1190 cells with addition of oxygen after varied time periods in lack of oxygen. Oxygen was added after 7, 14, 21, 28, and 35 days, and each time, the level of 1,4-dioxane decreased to zero. In the “56 days Anaerobic” condition, oxygen was never added during the experiment, and the 1,4-dioxane levels remained high.

Thus, it seems these scientists have found a very promising combination of cells—KB-1 and CB1190—that could potentially eliminate three cancer-causing chemicals from the groundwater in our environment. If many different protocols for bioremediation continue to be developed, bacteria could hopefully be used to break down many different chemicals to reverse the pollution in contaminated water and soil everywhere. Then, just as plants have evolved to convert our carbon dioxide back into oxygen, maybe one day bacteria could convert our chemical waste into food for them to grow, survive, and clean up our earth.